Cryogenic refrigerator

ABSTRACT

A cryogenic refrigerator according to an embodiment of the present invention is used in cooling a superconducting magnet. The cryogenic refrigerator includes a crankpin bearing of a scotch yoke. The crankpin bearing is made of a cylindrical nonmagnetic resin material. The crankpin bearing includes a cylindrical inner wall and a cylindrical outer wall. The cylindrical inner wall supports a crankpin. The crankpin is eccentrically attached to an output shaft of a motor slidably in a circumferential direction. The cylindrical outer wall contacts with an inner wall of a window portion of the scotch yoke. The cylindrical inner wall is made of another nonmagnetic resin material different from the nonmagnetic resin material of the cylindrical outer wall. Hardness of the cylindrical inner wall is higher than hardness of the cylindrical outer wall.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based upon and claims the benefit of priority of Japanese Patent Application No. 2011-059876, filed on Mar. 17, 2011, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to cryogenic refrigerators. Specifically, the present invention relates to cryogenic refrigerators used in cooling a superconducting magnet.

2. Description of the Related Art

A Gifford-McMahon (GM) refrigerator provided with a scotch yoke is known (see Japanese Unexamined Patent Application Publication No.H6-300378 for example). The scotch yoke converts a rotational motion of a motor to a linear reciprocating motion.

This GM refrigerator is mainly comprised of a compressor, a regenerator, an expander, and a switching valve. To produce ultra-low temperature, the GM refrigerator supplies a refrigerant gas compressed by the compressor to the expander through the switching valve and the regenerator, and expands in the expander the refrigerant gas cooled by the regenerator.

The expander is mainly comprised of a cylinder and a displacer reciprocated in the cylinder. The displacer is reciprocated in the cylinder by using the scotch yoke.

The scotch yoke is mainly comprised of a flat plate part having two axes protruding up and down, and a crankpin bearing rollably positioned in a rounded-rectangle-shaped window portion formed in the flat plate part. The crankpin bearing is configured to support a crankpin slidably in a circumferential direction. The crankpin is eccentrically attached to an output shaft of a motor.

SUMMARY OF THE INVENTION

A cryogenic refrigerator according to an embodiment of the present invention is a cryogenic refrigerator used in cooling a superconducting magnet including a crankpin bearing of a scotch yoke, wherein the crankpin bearing is made of cylindrical nonmagnetic resin material, and includes a cylindrical inner wall supporting a crankpin eccentrically attached to an output shaft of a motor slidably in a circumferential direction and a cylindrical outer wall contacting with an inner wall of a window portion of the scotch yoke.

In this way, the present invention is able to provide a cryogenic refrigerator suitable for cooling a superconducting electromagnet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a configuration example of a cryogenic refrigerator according to an embodiment of the present invention;

FIG. 2 is an exploded perspective view of a crank member, a scotch yoke, and a rotary valve in a housing;

FIG. 3 is an exploded perspective view of a rotary valve;

FIG. 4 is a diagram illustrating a relative relationship between a valve plate and a valve body;

FIGS. 5A and 5B are detail views of a crankpin bearing;

FIGS. 6A and 6B are detail views of a crankpin bearing;

FIGS. 7A, 7B, and 7C are diagrams each illustrating a relationship between a width of a cylindrical inner wall part and a width of a cylindrical outer wall part of a crankpin bearing;

FIGS: 8A, 8B, and 8C are detail views of a crankpin bearing;

FIGS. 9A, 9B, 9C, and 9D are detail views of a crankpin bearing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described by the reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention.

The crankpin bearing is typically made of iron with excellent hardness. If a GM refrigerator including a crankpin bearing made of a ferromagnetic material such as iron is used in cooling a superconducting magnet which generates an intense magnetic field, the GM refrigerator may cause an uneven wear of the crankpin bearing, because the crankpin bearing may be subjected to an external force from the superconducting magnet depending on placement of the GM refrigerator.

In consideration of the above, the present invention is designed to provide a cryogenic refrigerator suitable for cooling a superconducting electromagnet.

FIG. 1 is a sectional view of a configuration example of a cryogenic refrigerator according to an embodiment of the present invention. The cryogenic refrigerator 1 is, for example, a GM refrigerator. The cryogenic refrigerator mainly includes a compressor 2 and a refrigerator 3.

The compressor 2 takes in refrigerant gas (helium gas) from its low pressure side 2A, raises the pressure of the refrigerant gas, emits the refrigerant gas into its high pressure side 2B, and supplies the refrigerant gas into the refrigerator 3.

The refrigerator 3 is comprised of a housing 4, a cylinder 5, a motor 6, a crank member 7, a scotch yoke 8, a rotary valve 9, a two-stage displacer 10, a first cooling stage 11, and a second cooling stage 12.

The housing 4 has a refrigerant gas inlet port 13 connected to the high pressure side 2B of the compressor 2 and a refrigerant gas outlet port 14 connected to the low pressure side 2A of the compressor 2.

As illustrated, the cylinder 5 accommodates the two-stage displacer 10 so that the two-stage displacer 10 can slidably reciprocate in an up-and-down direction. A seal is positioned on the sliding contact portion.

The motor 6 is, for example, an electric motor. The motor 6 has an output shaft 24 coupled to the crank member 7.

Hereinafter, referring to FIGS. 2 and 3, the crank member 7, the scotch yoke 8, and the rotary valve 9 in the housing 4 will be explained in detail. FIG. 2 is an exploded perspective view of the crank member 7, the scotch yoke 8, and the rotary valve 9 in the housing 4. FIG. 3 is an exploded perspective view of the rotary valve 9.

The crank member 7 is coupled to the output shaft 24 of the motor 6 via a key connection. The crank member 7 has a crankpin 25 which is arranged eccentrically in relation to the output shaft 24 of the motor 6 and extends parallel to the output shaft 24.

The scotch yoke 8 has a horizontally long flat plate part 26, an upper shaft 27U, a lower shaft 27D, and a crankpin bearing 28. The lower shaft 27D is fixed to an upper part of the displacer 10. A rounded-rectangle-shaped window portion 34 is formed in the horizontally long flat plate part 26. The cylindrically-shaped crankpin bearing 28 is rollably placed inside the window portion 34.

The scotch yoke 8 receives the crankpin 25 slidably at an inner wall of the crankpin bearing 28.

Rotation of the scotch yoke 8 about the upper shaft 27U and the lower shaft 27D is restricted by a rotation stopping pin (not shown). The upper shaft 27U and the lower shaft 27D are supported by a pair of slide bearings 35U, 35D (see FIG. 1) which also serve as a seal material, respectively. Thus, the scotch yoke 8 is configured to be able to reciprocate in an up-and-down direction.

In this way, the scotch yoke 8 converts a rotational motion of the crank member 7 to an up-and-down reciprocating motion of the displacer 10.

The crankpin bearing 28 is preferably made of a nonmagnetic resin material.

The rotary valve 9 has a valve plate 29, valve body 30, a compression spring 31, and a fixing pin 32. The crankpin 25 is received in a crankpin engaging hole 33 formed in the valve plate 29.

The valve plate 29 has a low pressure side face 29A at the side facing the crankpin member 7 and a high pressure side face 29B facing the valve body 30 at the opposite side of the low pressure side face 29A.

The valve plate 29 also has an arc-like through-hole 29C axially extending between the low pressure side face 29A and the high pressure side face 29B, and a radially extending groove 29D formed on the high pressure side face 29B.

The valve body 30 has a first lateral face 30A closely contacting with the high pressure side face 29B, and a second lateral face 30B receiving the refrigerant gas from the compressor 2 at the opposite side of the first lateral face 30A.

The valve body 30 also has a central through-hole 30C extending between the first lateral face 30A and the second lateral face 30B, an arc-like groove 30D formed on the first lateral face 30A, a rectangular connecting hole 30E connected to the arc-like groove 30D and opening at an outer peripheral surface of the valve body 30, and a fixing hole 30F to which the fixing pin 32 is inserted.

The rectangular connecting hole 30E is connected through a gas flow channel 36 (see FIG. 1) in the housing 4 to a first space 17 at the displacer 10.

FIG. 4 is a diagram of a contact surface (the high pressure side face 29B and the first lateral face 30A) between the valve plate 29 and the valve body 30, viewed from a side of the valve body 30.

As shown in FIG. 4, the arc-like through-hole 29C has a shape formed by elongating a round hole, which is placed radially away from a center of the valve plate 29 at the same distance as a distance between the central through-hole 30C and the arc-like groove 30D, along a circumference of a circle with the same radius as the above distance by a certain arc angle.

The oval-shaped radially extending groove 29D on the valve plate 29 extends radially from the center of the valve plate 29, and has the same length as the distance between the central through-hole 30C and the arc-like groove 30D.

Similar to the arc-like through-hole 29C, the arc-like groove 30D on the valve body 30 has a shape formed by elongating a round hole, which is placed radially away from a center of the valve body 30 at the same distance as the distance between the central through-hole 30C and the arc-like groove 30D, along a circumference of a circle with the same radius as the above distance by a certain arc angle.

In this way, the valve plate 29 matches its central axis to a central axis of the valve body 30. The valve plate 29 slidingly rotates while bringing the high pressure side face 29B into contact with the first lateral face 30A, and causes the radially extending groove 29D to be continuously connected to the central through-hole 30C.

The valve plate 29 puts an intake valve into an “open” state and an exhaust valve into a “closed” state by connecting the radially extending groove 29D to the arc-like groove 30D, and puts the intake valve into a “closed” state and the exhaust valve into an “open” state by connecting the arc-like through-hole 29C to the arc-like groove 30D.

The valve plate 29 is supported by a bearing 37 (see FIG. 1) rotatably and axially immovably. The valve body 30 is fixed by the fixing pin 32 so that the valve body 30 may be non-rotatable in relation to the housing 4 and slidable in relation to the valve plate 29.

The compression spring 31 is a member to prevent the valve body 30 from being biased. The compression spring 31 pushes the valve body 30 against the valve plate 29.

Referring again to FIG. 1, components of the refrigerator 3 of the cryogenic refrigerator 1 will be explained.

The displacer 10 incorporates a first cool storage material 15 and a second cool storage material 16. The displacer 10 forms a first space 17 (a room air temperature space), a second space 18 (a first expansion space), and a third space 19 (a second expansion space) between the displacer 10 and the cylinder 5. The displacer 10 allows these three spaces to communicate through a first connecting hole 20, a second connecting hole 21, a third connecting hole 22, and a fourth connecting hole 23.

If the displacer 10 goes up, the first space 17 decreases its volume, while the second space 18 and the third space 19 increase their volumes.

The first cooling stage 11 and the second cooling stage 12 are formed by flanges outwardly extending from the cylinder 5, and are placed so that a low temperature generated in the cylinder 5 may be transferred to an object to be cooled.

Hereinafter, a normal cooling mode operation of the cryogenic refrigerator 1 will be explained.

Helium gas from the high pressure side 2B of the compressor 2 is fed into the housing 4 through the refrigerant gas inlet port 13. Then, the high pressure helium gas pushes the valve body 30 against the valve plate 29 in cooperation with the compression spring 31.

A major part of the helium gas reaches the radially extending groove 29D of the valve plate 29 through the central through-hole 30C of the valve body 30.

As shown in FIG. 3, the valve plate 29 is rotated by the motor 6 in a counter clockwise direction to connect the radially extending groove 29D and the arc-like groove 30D so that the refrigerant gas may be supplied to the first space 17. This state corresponds to the above described “open” state of the intake valve. The scotch yoke 8 and the displacer 10 start to go up if the intake valve comes into the “open” state.

After the intake valve comes into the “open” state, along with the rise of the displacer 10, the helium gas reaches the first space 17 through the central through-hole 30C, the radially extending groove 29D, the arc-like groove 30D, the rectangular connecting hole 30E, and the gas flow channel 36. Then, the helium gas reaches the second space 18 after being cooled by the first cool storage material 15, and then reaches the third space 19 after being cooled by the second cool storage material 16.

If the displacer 10 reaches a top dead point, the valve plate 29 disconnects the radially extending groove 29D from the arc-like groove 30D, and connects the arc-like through-hole 29C to the arc-like groove 30D.

As a result, the valve plate 29 shuts off a flow of the high pressure helium gas from the compressor 2 to the cylinder 5. On the other hand, the valve plate 29 puts the exhaust valve into the “open” state, and expands the high pressure helium gas in the second space 18 and the third space 19 along with the descent of the displacer 10.

The expanded helium gas cools down the first cool storage material 15 and the second cool storage material 16. Then, the helium gas passes through the gas flow channel 36, the rectangular connecting hole 30E, the arc-like groove 30D, and the arc-like through-hole 29C. Then, the helium gas passes through the vicinity of the scotch yoke 8 and the crank member 7, and returns to the low pressure side 2A of the compressor 2 through the refrigerant gas outlet port 14.

By repeating the above cooling mode operation, the cryogenic refrigerator 1 cools down the first cooling stage 11 and the second cooling stage 12.

Next, referring to FIGS. 5A and 5B, details of the crankpin bearing 28 of the scotch yoke 8 will be explained. FIGS. 5A and 5B are detail views of the crankpin bearing 28. FIG. 5A is an enlarged view of a region enclosed by a dotted line circle in FIG. 1. FIG. 5B is a sectional view taken along a line VB-VB in FIG. 5A. A sectional view taken along a line VA-VA in FIG.5B corresponds to FIG. 5A.

Preferably, the crankpin bearing 28 is made of a cylindrically-shaped nonmagnetic resin material. The crankpin bearing 28 has a cylindrical inner wall contacting the crankpin 25 and a cylindrical outer wall contacting an inner wall of the rounded-rectangular-shaped window portion 34 formed in the flat plate part 26.

The scotch yoke 8 is configured so that a contact frictional force between the cylindrical inner wall of the crankpin bearing 28 and the crankpin 25 may be less than a contact frictional force between the cylindrical outer wall of the crankpin bearing 28 and the flat plate part 26.

This is to allow the crankpin bearing 28 to roll on the inner wall of the window portion 34 while allowing the crankpin bearing 28 to slidably support the crankpin 25 on the cylindrical inner wall of the crankpin bearing 28.

Thus, the crankpin bearing 28 is, for example, integrally made of a fluorine resin material or the like which is hard and has excellent slidability.

The crankpin bearing 28 made of a nonmagnetic resin material has an effect of preventing a seizure between the crankpin bearing 28 and each of the crankpin 25 and the flat plate part 26 which are made of a nonmagnetic stainless steel.

The crankpin bearing 28 causes the outer peripheral surface of the crankpin 25 to slide on the cylindrical inner wall of the crankpin bearing 28. The crankpin 25 moves (revolves) along a circular path TR (see FIG. 5B) whose center is located on the output shaft 24 of the motor 6.

The crankpin bearing 28 frictionally contacts its cylindrical outer wall with the inner wall of the window portion 34 formed in the flat plate part 26. The crankpin bearing 28 rotates (spins) in a direction indicated by an arrow AR, and thus apparently moves (rolls) leftward within the window portion 34.

In this way, the crankpin bearing 28 rolls on the inner wall of the window portion 34 without sliding, while relatively rotating on the crankpin 25 in relation to the crankpin 25. That is, the crankpin bearing 28 rotates while revolving along the circular path TR. Thus, the crankpin bearing 28 apparently reciprocates right and left within the window portion 34.

By the above configuration, the cryogenic refrigerator 1 provided with the crankpin bearing 28 is able to reciprocate the displacer 10 up and down by using the scotch yoke 8 which is unaffected by an intense magnetic field generated by a superconducting electromagnet. Thus, the cryogenic refrigerator 1 is able to cool down the superconducting electromagnet adequately.

Next, referring to FIGS. 6A and 6B, a crankpin bearing 28 a which is another embodiment of the crankpin bearing of the scotch yoke 8 will be explained. FIGS. 6A and 6B are detail views of the crankpin bearing 28 a. FIGS. 6A and 6B correspond to FIGS. 5A and 5B, respectively.

The crankpin bearing 28 a is formed by two members, i.e., a cylindrical inner wall part 28 a 1 and a cylindrical outer wall part 28 a 2. The crankpin bearing 28 a is different from the crankpin bearing 28 in the respect that each of the two members is made of different nonmagnetic resin materials. The cylindrical inner wall part 28 a 1 and the cylindrical outer wall part 28 a 2 are non-rotatably bound to each other by using a known technology.

The nonmagnetic resin material of the cylindrical inner wall part 28 a 1 is selected to have higher slidability than the nonmagnetic resin material of the cylindrical outer wall part 28 a 2.

This is to allow the crankpin bearing 28 a to roll on the inner wall of the window portion 34 while allowing the crankpin bearing 28 a to slidably support the crankpin 25 on the cylindrical inner wall part 28 a 1.

For example, the cylindrical inner wall part 28 a 1 of the crankpin bearing 28 a is made of a high-hardness resin material with high slidability such as a fluorine resin material or the like (e.g., PTFE, PPS, etc.), and the cylindrical outer wall part 28 a 2 of the crankpin bearing 28 a is made of a low-hardness resin material (e.g., polyethylene, polypropylene, polyamide, etc.).

Thus, the crankpin bearing 28 a is able to reduce manufacturing cost in comparison to the crankpin bearing 28 which is integrally made of the fluorine resin material.

In this case, each of usage amounts of the high-hardness resin material and the low-hardness resin material, that is, each of thicknesses of the cylindrical inner wall part 28 a 1 and the cylindrical outer wall part 28 a 2 is decided accordingly in consideration of a functional aspect and a cost aspect.

The crankpin bearing 28 a may basically be made of the high-hardness resin material, and a surface of its cylindrical outer wall may be coated with the low-hardness resin material. This is to further improve characteristics of the crankpin bearing 28 a that the crankpin bearing 28 a rolls on the inner wall of the window portion 34 without spinning free, while slidably supporting the crankpin 25 on its cylindrical inner wall.

On the contrary, the crankpin bearing 28 a may basically be made of the low-hardness resin material, and a surface of its cylindrical inner wall may be coated with the high-hardness resin material. This is to reduce manufacturing costs by decreasing a usage amount of the expensive high-hardness resin material while maintaining the above characteristics of the crankpin bearing 28 a.

FIGS. 7A-7C are diagrams each illustrating a relationship between width W1 of the cylindrical inner wall part 28 a 1 and width W2 of the cylindrical outer wall part 28 a 2 of the crankpin bearing 28 a. FIGS. 7A-7C correspond to FIG. 5A, respectively.

FIG. 7A shows the case where the width W1 is equal to the width W2, FIG. 7B shows the case where the width W2 is widened to width W21, and FIG. 7C shows the case where the width W1 is narrowed to width W11.

As shown in FIG. 7B, widening the width W2 of the cylindrical outer wall part 28 a 2 to the width W21 has an effect of stabilizing a rolling of the crankpin bearing 28 a within the window portion 34.

As shown in FIG. 7C, narrowing the width W1 of the cylindrical inner wall part 28 a 1 to the width W11 has an effect of decreasing a usage amount of the expensive high-hardness resin material.

In this way, the crankpin bearing 28 a allows the width W1 of the cylindrical inner wall part 28 a 1 and the width W2 of the cylindrical outer wall part 28 a 2 to be set separately by means of its two-member configuration. Also, the crankpin bearing 28 a allows the width W1 and the width W2 to be selected more flexibly depending on a desired effect including the above effect.

Each of the width W1 of the cylindrical inner wall part 28 a 1 and the width W2 of the cylindrical outer wall part 28 a 2 may be wider or narrower than width W3 of the flat plate part 26. The width W1 of the cylindrical inner wall part 28 a 1 may be wider than the width W2 of the cylindrical outer wall part 28 a 2.

By the above configuration, the cryogenic refrigerator 1 provided with the crankpin bearing 28 a is able to reciprocate the displacer 10 up and down by using the scotch yoke 8 which is unaffected by an intense magnetic field generated by a superconducting electromagnet. Thus, the cryogenic refrigerator 1 is able to cool down the superconducting electromagnet adequately.

The cylindrical inner wall part 28 a 1, which is relatively susceptible to wear, is made of the high-hardness resin material, and the cylindrical outer wall part 28 a 2, which is relatively insusceptible to wear, is made of the low-hardness resin material. Thus, the crankpin bearing 28 a can reduce manufacturing cost while maintaining required wear resistant characteristics in comparison to the case where the crankpin bearing 28 a is entirely made of the high-hardness resin material.

Next, referring to FIGS. 8A-8C, a crankpin bearing 28 b which is yet another embodiment of the crankpin bearing of the scotch yoke 8 will be explained. FIGS. 8A-8C are detail views of the crankpin bearing 28 b. FIGS. 8A-8C correspond to FIGS. 5A, respectively.

Specifically, FIG. 8A shows that a shaft line of the crankpin 25 and a shaft line of the scotch yoke 8 (i.e., shaft lines of the upper shaft 27U and the lower shaft 27D) bisect each other at right angles.

FIG. 8B shows that the shaft line of the crankpin 25 is inclined at a degrees to a horizontal line, and that the shaft line of the crankpin 25 and the shaft line of the scotch yoke 8 do not bisect each other at right angles. FIG. 8C shows that the shaft line of the scotch yoke 8 is inclined at β degrees to a vertical line, and that the shaft line of the crankpin 25 and the shaft line of the scotch yoke 8 do not bisect each other at right angles.

The crankpin bearing 28 b is different from the crankpin bearings 28 and 28 a in the respect that a contour 28 b 1 of the cylindrical outer wall is convex, if it is viewed from a direction perpendicular to its axial direction (i.e., if the figure is viewed from the front).

For sake of clarity, the contour 28 b 1 is highlighted by a heavy line. Specifically, the contour 28 b 1 is represented by an arc constituting a part of a circle with a predetermined radius.

Such a shape of the contour 28 b 1 has an effect of preventing unwanted force from being exerted on the crankpin bearing 28 b in actuating the scotch yoke 8 even if the shaft line of the crankpin 25 and the shaft line of the scotch yoke 8 do not bisect each other at right angles.

For example, the crankpin bearing 28 b can contact the inner wall of the window portion 34 at a center of a width direction of the window portion 34 due to the convex shape of the contour 28 b 1 even in the case shown in each of FIGS. 8B and 8C. Thus, the crankpin bearing 28 b allows the scotch yoke 8 to continue a smooth up-and-down reciprocating motion while preventing unwanted force from being exerted on the crankpin bearing 28 b.

In this context, the “unwanted force” includes, for example, an excessive force exerted on the crankpin bearing 28 b by the window portion 34. If a shape of the contour 28 b 1 is linear as in the crankpin bearings 28 and 28 a, the crankpin bearing 28 b brings a surface of its cylindrical outer wall into contact with a surface of the inner wall of the window portion 34 at an angle. The excessive force is caused by this contact.

The “unwanted force” also includes an excessive force exerted on the crankpin bearing 28 b by the crankpin 25. If a shape of the contour 28 b 1 is linear, the crankpin bearing 28 b brings the surface of the cylindrical inner wall into contact with an outer surface of the crankpin 25 at an angle. The excessive force is caused by this contact.

In FIGS. 8A-8C, a width W4 of the crankpin bearing 28 b is wider than the width W3 of the flat plate part 26. However, the width W4 may be equal to or narrower than the width W3.

By the above configuration, the cryogenic refrigerator 1 provided with the crankpin bearing 28 b is able to reciprocate the displacer 10 up and down by using the scotch yoke 8 which is unaffected by an intense magnetic field generated by a superconducting electromagnet. Thus, the cryogenic refrigerator 1 is able to cool down the superconducting electromagnet adequately.

The crankpin bearing 28 b can also allow the scotch yoke 8 to continue a smooth up-and-down reciprocating motion while preventing unwanted force from being exerted on the crankpin bearing 28 b. Thus, the crankpin bearing 28 b is able to reduce or remove an abnormal noise caused by the unwanted force.

As in the crankpin bearings 28 and 28 a, the crankpin bearing 28 b is made of a nonmagnetic resin material, and the cryogenic refrigerator 1 provided with the crankpin bearing 28 b can be used in cooling a superconducting magnet. However, a feature concerning the contour 28 b 1 of the crankpin bearing 28 b is also advantageously applicable to the case where the cryogenic refrigerator 1 provided with the crankpin bearing 28 b is used in applications other than the cooling of the superconducting magnet. In this case, the crankpin bearing 28 b may be made of magnetic material.

Next, referring to FIGS. 9A-9D, crankpin bearings 28 c-28 e, each of which is yet another embodiment of the crankpin bearing of the scotch yoke 8., will be explained. FIGS. 9A-9D are sectional views of the crankpin bearings 2Bc-28 e, respectively.

FIG. 9A shows a sectional view of the crankpin bearing 28 b shown in FIGS. 8A-8C as a target for comparison.

As explained above, the contour 28 b 1 of the crankpin bearing 28 b is represented by an arc constituting a part of a circle with a predetermined radius.

The contour 28 b 1 can continuously bring the cylindrical outer wall of the crankpin bearing 28 b into contact with the inner wall of the window portion 34 at the center of the width direction of the window portion 34. A range of inclination angles, with which the crankpin bearing 28 b can deal, increases with decrease of the curvature radius of the contour 28 b 1. The inclination angle represents an inclination angle of the shaft line of the crankpin 25 in relation to a horizontal line, or an inclination angle of the shaft line of the scotch yoke 8 in relation to a vertical line.

FIG. 9B shows a sectional view of the crankpin bearing 28 c. A contour 28 c 1 of the crankpin bearing 28 c includes a linear portion at its center and curved portions on both sides of the linear portion.

Due to the linear portion, the contour 28 c 1 can increase a contact area between the cylindrical outer wall of the crankpin bearing 28 c and the flat plate part 26 in comparison to the contour 28 b 1 in FIG. 9A if the shaft line of the crankpin 25 and the shaft line of the scotch yoke 8 bisect each other at right angles. Thus, the contour 28 c 1 can further stabilize a rolling of the crankpin bearing 28 c.

Also, due to the curved portion, the contour 28 c 1 allows the scotch yoke 8 to continue a smooth up-and-down reciprocating motion while preventing unwanted force from being exerted on the crankpin bearing 28 c even if the shaft line of the crankpin 25 and the shaft line of the scotch yoke 8 do not bisect each other at right angles.

In that case, the flat plate part 26 and the crankpin bearing 28 c contact each other at an area closer to an edge of the inner wall of the window portion 34 in comparison to the contour 28 b 1 in FIG. 9A.

FIG. 9C shows a sectional view of the crankpin bearing 28 d. The crankpin bearing 28 d is mainly comprised of a body part 28 d 1 made of a nonmagnetic resin material. The body part 28 d 1 has the same convex contour as the contour 28 b 1 in FIG. 9A. Preferably, the nonmagnetic resin material is a low-hardness resin material.

A cylindrical inner wall of the body part 28 d 1 is coated with another nonmagnetic resin material. Preferably, a covering layer 28 d 2 coated with the another nonmagnetic resin material is made of a high-hardness resin material such as a fluorine resin material.

FIG. 9D is a sectional view of the crankpin bearing 28 e. The crankpin bearing 28 e is comprised of a cylindrical outer wall part 28 e 1 and a cylindrical inner wall part 28 e 2. The cylindrical outer wall part 28 e 1 is made of a nonmagnetic resin material having the same convex contour as the contour 28 b 1 in FIG. 9A. The cylindrical inner wall part 28 e 2 is made of another nonmagnetic resin material which is different from the nonmagnetic resin material in the cylindrical outer wall part 28 e 1.

The cylindrical outer wall part 28 e 1 is preferably made of a low-hardness resin material. The cylindrical inner wall part 28 e 2 is preferably made of a high-hardness resin material such as a fluorine resin material.

By the above configuration, each of the crankpin bearings 28 d and 28 e are able to achieve an effect due to the crankpin bearing 28 a (i.e., an effect due to a combined use of two kinds of resin materials) as well as an effect due to the crankpin bearing 28 b (i.e., an effect due to a contour shape including a curve).

Although the embodiments of the present inventions have been described in detail, the present inventions shall not be limited to the embodiments described above. It should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.

For example, in the above described embodiments, the crankpin bearings 28 a, 28 d, and 28 e are made of two kinds of resin materials, respectively. However, they may be made of more than two kinds of resin materials.

In that case, in the crankpin bearings 28 a, 28 d, and 28 e, a portion which does not come into contact with another part may be made of a low-hardness resin material, and each of portions which come into contact with other parts may be made of different high-hardness resin materials.

Also, in the crankpin bearings 28 a, 28 d, and 28 e, each of the portions which come into contact with other parts may be made of an identical high-hardness resin material.

The cryogenic refrigerators according to the embodiments of the present invention are particularly suitable for use with MRI applications because it does not generate a magnetic noise. 

1. A cryogenic refrigerator used in cooling a superconducting magnet, comprising a crankpin bearing of a scotch yoke, wherein the crankpin bearing is made of a cylindrical nonmagnetic resin material, and includes a cylindrical inner wall supporting a crankpin eccentrically attached to an output shaft of a motor slidably in a circumferential direction and a cylindrical outer wall contacting with an inner wall of a window portion of the scotch yoke.
 2. The cryogenic refrigerator as claimed in claim 1, wherein the cylindrical inner wall is made of another nonmagnetic resin material different from the nonmagnetic resin material of the cylindrical outer wall.
 3. The cryogenic refrigerator as claimed in claim 2, wherein hardness of the cylindrical inner wall is higher than hardness of the cylindrical outer wall.
 4. The cryogenic refrigerator as claimed in claim 1, wherein a contour of the cylindrical outer wall is convex, if the crankpin bearing is viewed from a lateral direction.
 5. The cryogenic refrigerator as claimed in claim 2, wherein a contour of the cylindrical outer wall is convex, if the crankpin bearing is viewed from a lateral direction.
 6. The cryogenic refrigerator as claimed in claim 3, wherein a contour of the cylindrical outer wall is convex, if the crankpin bearing is viewed from a lateral direction.
 7. The cryogenic refrigerator as claimed in claim 4, wherein the contour is represented by an arc.
 8. The cryogenic refrigerator as claimed in claim 5, wherein the contour is represented by an arc.
 9. The cryogenic refrigerator as claimed in claim 6, wherein the contour is represented by an arc.
 10. The cryogenic refrigerator as claimed in claim 4, wherein the contour includes a linear portion.
 11. The cryogenic refrigerator as claimed in claim 5, wherein the contour includes a linear portion.
 12. The cryogenic refrigerator as claimed in claim 6, wherein the contour includes a linear portion. 